How to generate NaN, -Infinity and +Infinity in ANSI C?

There is in C99, but not in previous standards AFAIK.

In C99, you'll have NAN and INFINITY macros.

From "Mathematics <math.h>" (§7.12) section

The macro INFINITY expands to a constant expression of type float representing positive or unsigned infinity, if available; ...

If you're stuck with ANSI C89, you're out of luck. See C-FAQ 14.9.

I don't know if this is standard or portable, but here's a start:

jcomeau@intrepid:/tmp$ cat test.c; make test; ./test
#include <stdio.h>
int main() {
 printf("%f\n", 1.0 / 0);
 printf("%f\n", -1.0 / 0);
 printf("%f\n", 0.0 / 0);
 return 0;
}
cc     test.c   -o test
test.c: In function ‘main’:
test.c:3: warning: division by zero
test.c:4: warning: division by zero
test.c:5: warning: division by zero
inf
-inf
-nan

Strangely enough, I can't get positive NaN using this naive approach.

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Also see this: http://www.gnu.org/s/hello/manual/libc/Infinity-and-NaN.html